The present invention relates to a circuit for detecting the delete patterns of an optical disc, and more particularly to a delete pattern detecting circuit suitable for detecting delete patterns in an optical disc of the WORM (write once read many) type device.
In the optical disc recording device, generally speaking, data is recorded by forming holes or pits in a track of the disc. However, the data that has once been thus formed cannot physically be eliminated or erased when it is desired to present new data, unlike magnetic disc memory systems. In the WORM type optical disc device, generally speaking, new data must be written in new data areas that have not been used previously for writing data. On the other hand, previously written data that is no longer desired, can be effectively deleted by overwriting delete patterns of long pits over top of the unnecessary recorded data pits, so that the delete pattern can be recognized from the ordinary data.
In the past, the formatting of optical discs have included a delete flag, wherein the delete flag is set whenever it is desired to delete the data in the sector corresponding to the delete flag. However, this delete flag is of short length, for example 1 bit, and therefore easily affected by dust and the like to falsely produce a delete signal so that valid data is falsely considered deleted. The previously described delete patterns represent an improvement over the delete flag in that the patterns are longer and less susceptible to false reading.
A system for performing such delete patterns is disclosed according to a document entitled "Explanatory Note On The Sector Format For Continuous Servo Method", June 1987, Continuous Servo working Group in Japan. Accordingly, the data recorded in an optical disc is deleted by providing overriding long pits or delete patterns other than the data recorded pattern on top of unwanted previously written data, for a constant period of time.